Red John is Agent Kirkland.
In season 5 episode 18, he kills the witness who knew Red John, and before that he asked him: "Do you recognize me?"
When he is asked by Lisbon in an older episode (season 5 episode 13) if he was at the Visualize farm, he doesn't answer.
He is the only person whom full name is unknown, so it's probable that he is Red John.
